Sagnac interferometer optical fibre current sensors of various types are well known. U.S. Pat. No. 5,677,622 granted to the University of Sydney as assignee of Ian G. Clarke discloses one such current sensor and it comprises a single sensing coil of spun single mode birefringent (“Hi-Bi”) optical fibre that is in use located about a current conductor, typically a large-current carrying busbar. Counter-propagating light beams are launched into the coil by way of a 3×3 coupler and a measure of the magnitude of current flow is detected as the phase shift between polarisation modes of the counter-propagated light beams.
Current measuring by known Sagnac interferometers is adversely affected by rotational movement of the sensing coil about a normal to the plane of the coil and it has been determined by the Inventor that a small rotational movement (created, for example, by a 50 Hz or a 60 Hz mechanical vibration) can produce a large phase shift in polarisation modes relative to that produced by a change in magnetic field and, hence, current magnitude.
The present invention in its primary form seeks to provide a sensing coil winding that facilitates nullification, or at least partial nullification, of the effects of rotational movement; that is, a sensing coil that provides for minimal sensitivity to rotational movement.